<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        /* nth-child 子元素必须是同一种元素
        nth-of-type 子元素元素类型不一样也可以选中（容错率高） */
        body :nth-child(2){
            background-color:blue;
        }
    </style>
</head>
<body>
    <div>div</div>
    <div>div</div>
    <div>div</div>
</body>
</html>